Okay, my first thought when I heard of this was "Oh, no". Then I read the piece, which states that Shane Black, who wrote the original movie (and none of its sequels) wrote a spec script, and that it involves Riggs, about to retire, pulling Murtaugh out of retirement to solve one final case, before they both officially head off into the sun. This sort of acknowledgment of the characters' age makes it sound legit, and since there are indeed cop films with older characters (DeNiro, Pacino, etc.), why not? Gibson and Glover are said to both be on board, though no deals have yet been signed.

Aren't spec scripts a bit of a no-no with the Writer's Guild? Their mantra being that you should not do for free what you can be paid for.
I thought Lethal Weapon 4 was supposed to wrap up the series. At least I got that impression from the way its end credits were done, with the titles posted over still pictures of the cast and crew.
And, would director Richard Donner come back? He's directed all four LW films, so (IMHO), he's as essential as Gibson or Glover to the franchise. Not to mention Rene Russo, Joe Pesci and Steve Kahan (who is Donner's cousin; IIRC, Donner casts him in all his films.) BTW, on the commentary for the LW 4 DVD, he said that that pic started shooting in January 1998 for release the following July, and it was edited in three weeks.

Presumably Black will be paid if it's used?

This is supposedly a go; both Gibson and Danny Glover are on board. Gibson is directing; supposedly he promised Richard Donner he would take over if Donner couldn't. (Donner passed away in 2021.) Personally, I think they're both "too old for this sh*t" and the movie probably will never happen. More on that here.